1. Field of the Invention
The present invention relates to an electron gun for use with a traveling wave tube, a klystron and the like, and more particularly, to a Pierce type electron gun which has a Wehnelt electrode (also referred to as a xe2x80x9cfocus electrodexe2x80x9d) for focusing an electron beam.
2. Description of the Related Art
A traveling wave tube and a klystron are electron tubes which rely on interaction of an electron beam emitted from an electron gun with a high frequency circuit for amplifying and oscillating microwaves. As illustrated in FIG. 1, for example, these electron tubes are each composed of electron gun 21 for emitting an electron beam; high frequency circuit 22 for promoting the interaction of the electron beam emitted from electron gun 21 with a high frequency signal (microwave); a collector 23 for capturing the electron beam delivered from high frequency circuit 22; and anode electrode 24 for guiding the electron beam emitted from electron gun 21 into high frequency circuit 22.
An electron beam emitted from electron gun 21 is introduced into high frequency circuit 22 by anode electrode 24, and travels within high frequency circuit 22 while it interacts with a high frequency signal applied to high frequency circuit 22. The electron beam delivered from high frequency circuit 22 is applied to collector 23 and captured by a collector electrode included in collector 23. In this event, high frequency circuit 22 delivers a high frequency signal which is amplified through the interaction with the electron beam.
While many types of electron guns are known for use with such traveling wave tubes and klystrons, a Pierce type electron gun has a Wehnelt electrode for focusing an electron beam, as one of such electron guns.
FIG. 2 is a lateral sectional view illustrating the structure of a conventional Pierce type electron gun.
As illustrated in FIG. 2, the conventional Pierce type electron gun comprises cathode 11 for emitting electrons; and a Wehnelt electrode 15 for focusing electrons emitted from cathode 11.
Cathode 11 is made of a discal porous tungsten base impregnated with an oxide (emitter material) of barium (Ba), calcium (Ca), aluminum (Al) or the like, and is bonded to cylindrical heater cap 12 made of molybdenum (Mo) or the like by welding or brazing to seal an open end thereof. Cathode 11 should be formed thick enough to endure the welding or brazing temperature and facilitate the bonding of cathode 11 to the cylindrical inner wall of heat cap 12 at a right angle. A heater, not shown, is disposed within heater cap 12 for applying thermal energy for emitting electrons from cathode 11.
Wehnelt electrode 15 is formed in a troidal shape having an opening at the center by cutting a metal material such as molybdenum, and bonded to one open end of Wehnelt support 14 formed in a cylindrical shape by welding or brazing.
Heater cap 12 mounted with cathode 11 is supported in Wehnelt supporter 14, for example, in a tripod structure, by metal supporters 16 made of tantalum (Ta), molybdenum (Mo), molybdenum-rhenium (Moxe2x80x94Re) alloy, iron-nickel-cobalt alloy (koval:Kv), or the like, and fixed at a position at which an electron emitting surface of cathode 11 is substantially coplanar with the surface of Wehnelt electrode 15. As illustrated in FIG. 2, Wehnelt electrode 15 is formed such that its surface closer to anode electrode 19 has an angle of approximately 67.5 degrees to the outermost shell of electron beam 18 (referred to as the xe2x80x9cPierce anglexe2x80x9d).
Wehnelt supporter 14, which contains heat cap 12 mounted with cathode 11, is securely fixed within an electron gun housing for vacuum encapsulation.
In the foregoing Pierce type electron gun, Wehnelt electrode 15 is applied with the same potential as cathode 11 to make a focusing action which shapes electrons emitted from cathode 11 into a beam which is introduced into the high frequency circuit (see FIG. 1) by anode electrode 19.
In the conventional Pierce type electron gun, an electrode spacing between the cathode and Wehnelt electrode, and an electrode spacing between the cathode and anode electrode, i.e., perveance must be made consistent with design values with high accuracy in order to focus electrons emitted from the cathode within a desired beam diameter. Particularly, it is critical to satisfy a dimensional accuracy for a narrow spacing between the cathode and Wehnelt electrode.
A large perveance between the cathode and Wehnelt electrode would give rise to collision of electrons emitted from the cathode with the anode electrode, and a varying diameter of an electron beam within the high frequency circuit to cause uneven interaction with a high frequency signal, resulting in increased power consumption, degraded amplification performance, and the like of the traveling wave tube.
In the structure of the conventional Pierce type electron gun illustrated in FIG. 2, it is quite difficult to integrally form the Wehnelt electrode, Wehnelt supporter and metal supporters through cutting operations, so that the Wehnelt electrode, heater cap, metal supporters and Wehnelt supporter are separately formed and bonded to one another by welding, brazing or the like. Thus, the conventional Pierce type electron gun has disadvantages of a larger number of parts and a long time required for assembling. In addition, since a larger number of parts causes an increase in dimensional errors of respective parts, mounting errors and distortion associated with bonding, and the like, it is difficult to limit the perveance of the cathode and Wehnelt electrode within a predetermined value.
It is therefore an object of the present invention to provide a Pierce type electron gun which prevents an increase in power consumption of a heater and an increase in perveance, and is composed of a fewer number of parts to facilitate its assembly.
To achieve the above object, an electron gun according to the present invention has a cathode for emitting electrons, a heater cap which contains a heater for applying the cathode with thermal energy for emitting electrons, a retainer for securing the cathode on the heater cap by holding the peripheral edge of the cathode to the heater cap, and a cylindrical Wehnelt supporter that has a Wehnelt electrode for focusing an electron beam formed in such a shape that an average angle of the surface thereof with respect to an outermost shell of said electron beam matches a Pierce angle, and three or more heater cap supporters for securely supporting said heater cap at a position at which an electron emitting surface of said cathode and an opening formed through said Wehnelt electrode satisfy a predetermined perveance.
Thus, the number of parts is reduced by integrally forming the Wehnelt supporter, Wehnelt electrode and heater cap supporters, resulting in a reduction in dimensional error of each part, mounting errors and distortion associated with bonding, and the like. Consequently, the perveance of the cathode and Wehnelt electrode is readily limited within a predetermined value. In addition, since the Wehnelt supporter including the Wehnelt electrode and heater cap supporters can be formed through pressing, less time is required for machining respective parts and assembling these parts into the Wehnelt supporter, and the cost is also reduced for the electron gun.
Also, in the present invention, each of the heater cap supporters is formed by cutting out the cylindrical side surface of the Wehnelt supporter in a strip shape except for one short side. In this event, the short side, left uncut, is one of sides parallel with the circumferential direction of the Wehnelt supporter, which is closer to the Wehnelt electrode. By thus forming the heater cap supporters in a strip shape except for the side closer to the Wehnelt electrode, heat radiated from the heater cap is prevented from leaking from strip-shaped openings formed through the side surface of the Wehnelt supporter, thereby saving the power consumption of the heater.
Further, in the present invention, the retainer has one end, which is brought into engagement with the cathode, in a folded shape or an arcuate shape. Since the thus shaped retainer is less susceptible to deformation due to the inflated cathode by the heat from the heater, the retainer maintains a sufficient force for retaining the cathode to prevent the cathode from shifting.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings, which illustrate examples of the present invention.